Boule oscillation patterns in methods of producing fused silica glass

Abstract

Oscillation/rotation patterns for improving the off-axis homogeneity of fused silica boules are provided. The patterns reduce the optical effects of periodic off-axis striae by decreasing the ratio of Δn striae to Δz striae , where Δn striae is the average peak-to-valley magnitude of the striae and Δz striae is their average peak-to-peak period. The oscillation/rotation patterns have long repeat periods and cause soot-producing burners to trace out substantially spiral-shaped patterns on the surface of the boule.

What is claimed is: 
     
       1. A method for forming a silica-containing body comprising: (a) providing a source of soot particles;   (b) collecting the soot particles to form the body, said collection causing the thickness of the body to increase; and   (c) providing relative oscillatory motion between the source and the body as the soot particles are collected, said oscillatory motion being orthogonal to the thickness of the body and having a repeat period;   said collection and oscillation being performed so that the thickness of the body increases by at least about 0.15 mm during the repeat period.   
     
     
       2. The method of claim 1 wherein the thickness of the body increases by at least about 0.2 mm during the repeat period. 
     
     
       3. A method for forming a silica-containing body comprising: (a) providing a source of soot particles;   (b) collecting the soot particles to form the body, said collection causing the thickness of the body to increase; and   (c) providing relative oscillatory motion between the source and the body as the soot particles are collected, said oscillatory motion being orthogonal to the thickness of the body and having a repeat period of at least about eight minutes.   
     
     
       4. The method of claim 3 wherein the repeat period is about 10 minutes. 
     
     
       5. A method for forming a silica-containing body comprising: (a) providing a source of soot particles;   (b) collecting the soot particles to form the body, said soot particles being consolidated to form a viscous fluid as they are collected, said collection causing the thickness of the body to increase; and   (c) rotating the body and providing relative oscillatory motion between the source and the body as the soot particles are collected, said oscillatory motion being orthogonal to the thickness of the body and said rotation and oscillatory motion being such that the projection of the source onto the body traces out a path which is substantially a spiral.   
     
     
       6. The method of claim 5 wherein the path is a pure spiral. 
     
     
       7. The method of claim 5 wherein the source comprises a plurality of soot producing burners and each burner traces out a path which is substantially a spiral. 
     
     
       8. The method of claim 5 wherein the oscillatory motion has a repeat period and the collection and oscillation are performed so that the thickness of the body increases by at least about 0.15 mm during the repeat period. 
     
     
       9. The method of claim 8 wherein the thickness of the body increases by at least about 0.2 mm during the repeat period. 
     
     
       10. The method of claim 5 wherein the oscillatory motion has a repeat period of at least about eight minutes. 
     
     
       11. The method of claim 10 wherein the repeat period is about 10 minutes. 
     
     
       12. A blank made from a silica-containing body made by the method of claim 1, 3, or 5, wherein the blank has: (a) a Δn striae  /Δz striae  value which is less than or equal to about 2.0×10 -8  mm -1 , where Δn striae  and Δz striae  are the average peak-to-valley magnitude and the average peak-to-peak period, respectively, of the blank's off-axis striae; and   (b) a z-axis homogeneity Δn which is less than or equal to 1.0×10 -6  for a blank size greater than or equal to 125 millimeters.   
     
     
       13. The blank of claim 12 wherein the blank contains a dopant. 
     
     
       14. An optical element made from the blank of claim 12. 
     
     
       15. A method for reducing off-axis periodic striae in the index of refraction of a blank formed from a silica-containing body, said off-axis periodic striae having an average peak-to-valley magnitude Δn striae  and an average peak-to-peak period Δz striae , said method comprising: (a) providing a source of soot particles;   (b) collecting the soot particles to form the body, said collection causing the thickness of the body to increase; and   (c) providing relative oscillatory motion between the source and the body as the soot particles are collected, said oscillatory motion being orthogonal to the thickness of the body and having a repeat period which is sufficiently long so that Δn striae  /Δz striae  for the blank is less than or equal to about 2.0×10 -8  mm -1 .   
     
     
       16. The method of claim 15 wherein the repeat period is sufficiently long so that Δn striae  /Δz striae  for the blank is less than or equal to about 1.5×10 -8  mm -1 . 
     
     
       17. The method of claim 15 wherein the blank has a z-axis homogeneity Δn which is less than or equal to 1.0 ×10 -6  for a blank size greater than or equal to 125 millimeters. 
     
     
       18. The method of claim 16 wherein the blank has a z-axis homogeneity Δn which is less than or equal to 1.0 ×10 -6  for a blank size greater than or equal to 125 millimeters. 
     
     
       19. The method of claim 15 wherein the blank has a z-axis homogeneity Δn which is less than or equal to 0.5 ×10 -6  for a blank size greater than or equal to 125 millimeters. 
     
     
       20. The method of claim 16 wherein the blank has a z-axis homogeneity Δn which is less than or equal to 0.5 ×10 -6  for a blank size greater than or equal to 125 millimeters. 
     
     
       21. The method of claim 17, 18, 19, or 20 wherein the blank size is greater than or equal to 150 millimeters. 
     
     
       22. The method of claim 17, 18, 19, or 20 wherein the blank size is greater than or equal to 200 millimeters. 
     
     
       23. The method of claim 15 wherein the body is rotated and the rotation and the oscillatory motion causes the projection of the source onto the body to trace out a path which is substantially a spiral. 
     
     
       24. The method of claim 23 wherein the path is a pure spiral. 
     
     
       25. The method of claim 23 wherein the source comprises a plurality of soot producing burners and each burner traces out a path which is substantially a spiral. 
     
     
       26. A blank made by the method of claim 16, wherein the blank has: (a) a Δn striae  /Δz striae  value which is less than or equal to about 2.0×10 -8  mm -1  ; and (b) a z-axis homogeneity Δn which is less than or equal to 1.0×10 -6  for a blank size greater than or equal to 125 millimeters.   
     
     
       27. The blank of claim 26 wherein the blank contains a dopant. 
     
     
       28. An optical element made from the blank of claim 26. 
     
     
       29. A fused silica blank which has: (a) a Δn striae  /Δz striae  value which is less than or equal to about 2.0×10 -8  mm -1 , where Δn striae  and Δz striae  are the average peak-to-valley magnitude and the average peak-to-peak period, respectively, of the blank's off-axis striae; and   (b) a z-axis homogeneity Δn which is less than or equal to 1.0×10 -6  for a blank size greater than or equal to 125 millimeters.   
     
     
       30. The fused silica blank of claim 29 wherein the blank's Δn striae  /Δz striae  value is less than or equal to about 1.5×10 -8  mm -1 . 
     
     
       31. The fused silica blank of claim 29 wherein the blank has a z-axis homogeneity Δn which is less than or equal to 0.5×10 -6  for a blank size greater than or equal to 125 millimeters. 
     
     
       32. The fused silica blank of claim 30 wherein the blank has a z-axis homogeneity Δn which is less than or equal to 0.5×10 -6  for a blank size greater than or equal to 125 millimeters. 
     
     
       33. The fused silica blank of claim 30, 31, 32 or 32 wherein the blank size is greater than or equal to 150 millimeters. 
     
     
       34. The fused silica blank of claim 30, 31, 32 or 32 wherein the blank size is greater than or equal to 200 millimeters. 
     
     
       35. An optical element made from the blank of claim 29. 
     
     
       36. A lens element made from the blank of claim 29. 
     
     
       37. A prism made from the blank of claim 29.